Decomposition of the Benzyl Radical: Quantum Chemical and Experimental (Shock Tube) Investigations of Reaction Pathways

Decomposition of the benzyl radical at a range of temperatures, ∼1450−1650 K, has been investigated using ab initio quantum chemical and experimental (shock tube) techniques. Four possible decomposition mechanisms are considered: (a) via a norbornadienyl intermediate, (b) via a cycloheptatrienyl intermediate, (c) via direct ring opening, and (d) via a 6-methylenebicyclo[3.1.0]hex-3-en-2-yl (MBH) intermediate. On the basis of the quantum chemical calculations, mechanisms c and d are found to be the dominant reaction channels. A theoretically derived rate constant for the overall disappearance of benzyl is k = 10 16.6±0.3 exp(−97 ± 3 kcal mol -1 / RT ) s -1, in reasonable agreement with that obtained in previous studies. The experiments were carried out by shock heating benzyl bromide to temperatures between 1050 and 1650 K, followed by analysis of the spectral components of benzyl bromide, benzyl, and benzyl “fragments”. The rate constants derived from these experiments by using a simple two-step kinetic model are in good agreement with the theoretical values.